1. Field of the Invention
The present invention relates to a transmission line switching system, particularly, it relates to a transmission line switching system for taking countermeasures against faults occurring in a transmission cable between remote locations (for example, between cities) through a plurality of terminal repeaters.
2. Description of the Related Art
Transmission lines used for data transmission, telephone circuits, and the like are constituted by, for example, an optical fiber cable or a coaxial cable as the transmission cable. Recently, optical fiber cables are widely utilized as a transmission cable due to the high speed and large volume data transmission obtained thereby.
In general, such a transmission cable is constituted by main cables and a spare cable (or cables). The spare cable is used in an emergency and is usually bundled together with the main cables as one transmission cable.
The most frequent type of transmission cable fault is disconnection of the cable, particularly, a complete disconnection of an optical fiber cable. Such a complete disconnection occurs, for example, during construction or when the cable is damaged by flood water, since an optical fiber cable is very thin and is not bendable.
When complete disconnection of the cable occurs, the transmission line must be switched to another transmission line, since the spare cable is also disconnected.
A problem occurs, however, in switching the transmission line, particularly, a transmission line connected by a plurality of terminal repeaters each having an intermediate relay function, as explained hereinafter.
Regarding transmission line switching systems, Japanese Unexamined Patent Publication (Kokai) No. 54-127615 discloses a switching system of a communication line for effectively utilizing a plurality of spare lines by selecting change-over switches; Japanese Unexamined Patent Publication (KOKAI) No. 59-175224 discloses a transmission line switching system for effectively switching between main lines, and between a main line and a spare line in accordance with the priority of the information; and Japanese Unexamined Patent Publication (KOKAI) No. 62-219720 discloses a transmission line switching system for effectively switching a plurality of spare lines.
Nevertheless, the above documents do not disclose a loop back function according to the present invention.